1. Field of the Invention
The present invention relates, in general, to an optical system and, more particularly, to an optical system for allowing the superposition of an electronic image formed by a projection device on an image coming from external scenery generated by a night vision goggle system.
2. Description of Related Art
Night vision goggles have been employed by the military services around the world for many years. A typical use for such goggles is navigation and situational awareness, for which the industry standard is to provide a 40° circular field of view (FOV). For use in night time or other similar very low light level conditions, the goggles are equipped with fast objective lenses where the entrance aperture diameter is nearly the same dimension as the lens focal length. The larger the lens aperture diameter, the more light gets through and hence the brighter the image. However, due to optical design complexity, the desire for small size and weight, and the need to reduce stray light reflections, most goggle objective lenses have a large degree of vignetting whereby the entering ray bundle diameters for the edges of the field of view are smaller than the ray bundle diameters for the central field of view.
With reference to FIG. 1, military specifications typically allow a “relative illumination” value of 40%, meaning the edge fields may have only 40% of the brightness as the central fields of view. In practice, the edge field vignetting is typically such that the light rays do not pass through the center of the outermost lens surface. This is a widely known phenomena when the “pinhole cap” is attached to a goggle so that it can be used in daytime conditions or when an increased depth of field is needed. The cap centers the pinhole on the central axis of the first lens surface, and substantially reduces the incoming ray bundle diameters for all fields of view. However, the edge field ray bundles are often completely obscured by the cap, and thus will never “see” through the pinhole. From an observer's point of view, the total field of view of the goggle is thereby reduced.
The objective lens vignetting problem discussed hereinabove also created difficulties for prior art systems that utilized an external projector attached to the goggles in order to import either symbology or full video overlay. A typical example of this kind of device is the Aviator's Night Vision Imaging System Head Up Display (ANVIS-HUD) as built by Elbit Systems, Inc. and which has been fielded by the United States military for many years. Another similar application which has recently emerged is the Clip-on Thermal Imager (COTI). This application utilizes an extra infrared camera that is attached to a standard night vision goggle and the camera image is projected in through the objective lens in much the same manner as the ANVIS-HUD. In all cases, the projector system is comprised of a display source which emits light into a set of collimation optics. The exit pupil of the collimation optics is then folded into the direction of view of the goggle objective lens. The folding may be via a small mirror or a coated dielectric surface such as may be found in a prism or plate beamsplitter. Currently, none of these projection systems features the ability to project an exterior image such that it completely fills the available 40° circular field of view of the goggle.
Much like the “Pinhole Cap” problem discussed hereinabove, the goggle objective lens vignetting would drive a conventional projector approach to have an exceptionally large projection exit pupil, which in turn would make the projection optic assembly extremely bulky and heavy. Typical performance based on literature surveys for all of the prior art systems is fields of view ranging from 20° to 32°. Even at the maximum projected field of 32° circular, the amount of visual area lost, compared to the available 40° of the normal goggle, is a significant 36%.
Accordingly, a need exists for a projection display attachment for a standard night vision goggle system that provides a 40° field of coverage without a substantial increase in size and weight of the standard night vision goggle system.